The invention is related to a screw actuator, comprising a nut and a screw each comprising helical grooves, said nut and screw engaging each other by means of rollers or balls which are in contact with said grooves, a lubricant being provided for the grooves and rollers or balls, and means which are integrated in the actuator for replenishing the lubricant for the grooves and rollers or balls of the actuator.
Such screw actuator is disclosed in U.S. Pat. No. 5,809,838. Said actuator comprises an oil reservoir in the gap between the races of the actuator support bearing.
The object of the invention is to provide an actuator in which the means for replenishing the lubricant can be applied in an easier and more versatile way. This object is achieved in that the means for replenishing the lubricant comprise a separate module or modular reservoir for containing an amount of lubricant, said module being held within a bore in the screw and being provided with means for feeding the lubricant to at least the helical grooves of the nut and/or screw.
Such separate, self-contained module is easy to install in an appropriately prepared actuator. Also, replacement of the module can be achieved in a simple way.
The screw and/or nut may comprise a pump means for expelling an amount of lubricant from the reservoir onto the grooves and rollers or balls.
The pump means may be driven in various ways. For instance, the pump means may be drivable by means of a relative displacement between the screw and the nut.
Alternatively, the pump means may be drivable by means of a gas pressure. Also, the pump means may be electrically drivable, control means being provided for controlling said electrically drivable pump means, e.g. a piezo-electric drive means.
Such piezo-electric drive means engages or comprises a piston, said piston being accommodated in the module.
According to a relatively simple embodiment, one of the nut and screw comprises an abutment, and the other comprises a pump means which is drivable by engagement with said abutment. Said abutment and the pump means engage each other upon approachment of the fully retracted position of the screw with respect to the nut.
In this embodiment, the grooves receive a fresh amount of lubricant upon replacement of the brake pads, after they have been worn out. Upon fitting the new, unworn brake pads, the screw actuator is pushed back to its original start position, thus bringing the pump means in engagement with the abutment. As a result, an amount of lubricant is driven from the reservoir towards the grooves.
Other ways of lubricating the grooves are possible as well. For instance, the grease reservoir may contain a screw pump and at least one grease outlet, said screw pump being arranged for displacing the grease through said outlet under the influence of a drive means actuatable through the relative displacement of nut and screw.
The screw actuator may comprise a sensor which detects a condition indicative for replenishment of lubricant. The sensor may be integrated in the bearing, or may be separately incorporated in the actuator. Said sensor emits a signal to a control unit, which in turn may actuate the pump means.
The invention is furthermore related to a brake calliper for a disc brake comprising a housing which carries two opposite brake pads enclosing a gap for accommodating a brake disc, said brake pads being displaceable towards and from each other by means of a screw actuator as described before, said screw actuator comprising a nut and a screw one of which is rotatably supported with respect to the housing by means of a rolling element bearing, said nut and screw comprising helical grooves and engaging each other by means of rollers or balls which are in contact with said grooves, a lubricant being provided for the grooves and rollers or balls.
According to the invention, means are provided which are integrated in the actuator for replenishing the lubricant for the grooves and rollers or balls of the actuator. Said means for replenishing the lubricant comprises a module which is connected to at least the screw threaded grooves of the nut or screw.
The grease may be fed towards the grooves by means of bleeding, or pumping. The pumping action may take place in a mechanical way, e.g. by means of a screw pump, or by a hybrid pump such as a piezo-electric pump. Also, the pump may be controlled on the basis of a detection signal from a sensor, indicating the need for regreasing the grooves.
The actuator may comprise either a ball screw or a roller screw.
Moreover, either the screw or the nut may be rotating.